Abstract : A review is presented of previous attempts to correlate earthquake occurrence with various other natural phenomena. These early studies were generally based on insufficient data, and therefore did not result in correlations that were acceptable to the scientific community. Continuous, highly sensitive monitoring of the 1966 Truckee, California, earthquake sequence presented the first opportunity to study rates of occurrence using a large number of observations collected in a small area. Time series analysis of 390 consecutive hours of data, totaling 10,550 earthquakes, yielded periodic components of occurrence of 12 1/2 and 25 hour periods; the known periods of earth tides. Crosscorrelation of aftershock frequencies with a Los Angeles earth-tide recording gave a correlation coefficient which was significant at the 95% level of confidence. (Author)